HIGHLY RESPONSIVE III-V PHOTODETECTORS USING ZnO:Al AS N-TYPE EMITTER

ABSTRACT

A photodiode includes a p-type ohmic contact and a p-type substrate in contact with the p-type ohmic contact. An intrinsic layer is formed over the substrate and including a III-V material. A transparent II-VI n-type layer is formed on the intrinsic layer and functions as an emitter and an n-type ohmic contact.

BACKGROUND

1. Technical Field

The present invention relates to photodiodes, and more particularly tophotodiodes with a II-VI n-type layer to reduce fabrication steps anddevice complexity.

2. Description of the Related Art

Conventional InGaAs photodiodes include a p-type ohmic contact on oneside of a p+ substrate (InP). An InGaAs intrinsic layer is formed on thesubstrate opposite the p-type ohmic contact. An n-type window layer(e.g., InP (Si doped)) is formed on the InGaAs layer, and an n+ contactlayer (e.g., InGaAs (Si doped)) is formed on the window layer. An n-typeohmic contact is formed over the contact layer.

While this structure (n-i-p) provides good performance, there are someoperational issues with the photodiode. For example, light absorption atthe n+ contact layer results in device losses due to low band gap andlow doping concentration in the n+ contact layer. In addition, darkcurrent of the n-i-p photodiode is high. Dark current is the relativelysmall electric current that flows through a photosensitive device whenno photons are entering the device. Incident radiation may enter fromthe n-type ohmic contact side. This n-type ohmic contact is generallycomprised on an opaque metal and therefore needs a reduced footprint toreduce shadowed areas. The shadow areas reduce the performance of thedevice.

SUMMARY

A photodiode includes a p-type ohmic contact and a p-type substrate incontact with the p-type ohmic contact. An intrinsic layer is formed overthe substrate and including a III-V material. A transparent II-VI n-typelayer is formed on the intrinsic layer and functions as an emitter andan n-type ohmic contact.

Another photodiode includes a p-type ohmic contact, a p+ III-V substratein contact with the p-type ohmic contact and a III-V intrinsic layerformed over the substrate. A single layer emitter is formed on theintrinsic layer and includes an Al doped ZnO. The single layer emitterfunctions as at least an emitter and an n-type ohmic contact. The Aldoped ZnO includes a thickness of less than 150 nm.

A method for forming a photodiode includes providing a p-type ohmiccontact, a p-type substrate in contact with the p-type ohmic contact andan intrinsic layer formed over the substrate and including a III-Vmaterial; and forming a transparent II-VI n-type layer on the intrinsiclayer to function as at least an emitter and an n-type ohmic contact.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a photodiode with a single layeremitter structure in accordance with the present principles;

FIG. 2 is a diagram showing ZnO:Al contact resistivity by plottingresistivity (Ohm-cm²) versus length (microns);

FIG. 3 is a diagram showing absolute value of current (mA) versusvoltage (V) for an n+ ZnO:Al photodiode in accordance with the presentprinciples and an n+ InGaAs photodiode in accordance with a conventionalstructure; and

FIG. 4 is a block/flow diagram showing a method for making a photodiodein accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, a photosensitive device, suchas a photodiode is provided that simplifies conventional photodiodes. AII-VI material, such as ZnO, is employed to replace three layers of theconventional device with a single thinner layer. The three layers of theconventional device that are replaced include the n-type semiconductorwindow layer, the contact layer (n+) and the n-type ohmic contact.Embodiments in accordance with the present principles provide manyadvantages over the conventional structure. For example, a photodiode inaccordance with the present embodiments generates less dark current,provides greater transparency for incident radiation and does notinclude metal shadowing areas since the II-VI material is transparentfor incoming radiation. The II-VI material also provides an extremelylow contact resistance.

It is to be understood that the present invention will be described interms of a given illustrative architecture and photovoltaic stack;however, other architectures, structures, substrates, materials andprocess features and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for a photovoltaic device may be created for integrated circuitintegration or may be combined with components on a printed circuitboard. The circuit /board may be embodied in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips or thephotolithographic masks used to fabricate chips or photovoltaic devices,the designer may transmit the resulting design by physical means (e.g.,by providing a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication ofphotovoltaic devices and/or integrated circuit chips with photovoltaicdevices. The resulting devices/chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged devices/chips), as a bare die, or in a packagedform. In the latter case, the device/chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case, thedevices/chips are then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips, ranging from toys, energy collectors, solar devices and otherapplications including computer products or devices having a display, akeyboard or other input device, and a central processor. Thephotovoltaic devices described herein are particularly useful forphotosensors for electronic devices, homes, buildings, vehicles, etc.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., InGaAs or ZnO. These compoundsinclude different proportions of the elements within the compound, e.g.,InGaAs includes In_(x)Ga_(1-x)As, where x is less than or equal to 1, orZnO includes Zn_(x)O_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound, such as, e.g.,AlInGaAs, and still function in accordance with the present principles.The compounds with additional elements will be referred to herein asalloys.

The present embodiments may be part of a photovoltaic device or circuit,and the circuits as described herein may be part of a design for anintegrated circuit chip, a solar cell, a light sensitive device, etc.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a cross-sectional view of aphotodiode 10 is illustratively shown in accordance with the presentprinciples. The photodiode 10 includes a p-type ohmic contact 12 formedon a back of the photodiode 10. The ohmic contact 12 is formed on a p+doped substrate 14. The p+ doped substrate 14 preferably includes aIII-V material, such as InP, although other materials may be employed,e.g., InGaAs, GaAs, etc.

An intrinsic layer 16 is formed on the substrate 14. The intrinsic layer16 may include a III-V material. In one embodiment, the intrinsic layer16 includes InGaAs, and more specifically, In₅₃Ga₄₇As. The intrinsiclayer 16 may have a thickness of about 1 micron, although otherthicknesses are contemplated.

In accordance with particularly useful embodiments, an n-type layer 18is formed on the intrinsic layer 16. The n-type layer 18 includes aII-VI material, such as ZnO, ZnS, ZnSe, CdS, CdTe, etc. In oneembodiment, the n-type layer 18 is transparent and may include ZnO,indium tin oxide (ITO), indium zinc oxide (IZO), etc. In one usefulembodiment, the n-type layer 18 includes Al doped ZnO (ZnO:Al or AZO).The n-type layer 18 may include a thickness of between about 50 nm toabout 200 nm, and preferably about 100 nm. The thickness of the n-typelayer 18 should balance between transmissivity of incident radiation(e.g., visible light) and conductivity. The thicker the n-type layer 18the less transmission and the greater the conductivity.

The n-type layer 18 is thinner than the conventional layers employed onthe n-type side of a conventional diode that it replaces. Theconventional diode employs a window layer (e.g., having a thickness ofabout 200-300 nm), a contact layer (e.g., having a thickness of about20-50 nm) and an n-type ohmic contact (e.g., greater than 200 nm). Theconventional layers include a nominal thickness of between about 420 nmto about 550 nm or greater. In accordance the present principles, thewindow layer, contact layer and the n-type ohmic contact are replacedwith a single n-type layer 18. The n-type layer 18 (which does not needan additional metal contact layer) may include a thickness of 100 nm,which is 4-5 (or more) times thinner than the conventional n-type sideof a photodiode. In addition, conductivity is maintained or improved anddark current is reduced (due at least to reducing the number ofinterfaces in the n-type side). Metal shadowing area (e.g., due themetal ohmic contact) is reduced or eliminated, transmission of light isimproved (due at least to the reduced thickness and the elimination ofmetal shadowing) and the structure is simplified making it more costeffective and less complex to fabricate. In particularly usefulembodiments, the n-type layer 18 may include a thickness of about 150nm. A thinner n-type layer 18 (emitter) is preferred. The n-type layer18 provides a single layer emitter structure.

The formation of ZnO:Al also tends to be easier than III-V materials.For example, instead of epitaxial growth processes (e.g., for n+InGaAs), ZnO:Al may be formed using atomic layer deposition (ALD),although other processes may be employed. This permits a doped layerwith less surface damage. Materials like Al may be formed directly onthe ZnO and be annealed to cause diffusion of the Al to dope the ZnO.

In accordance with the present principles, a range of n-doping in ZnO oflayer 18 is up to 2 atomic percent (e.g., ˜5×10²¹ /cm³). ZnO dopants mayinclude Al, B, Ga, In, etc., with Al:ZnO being preferred. ZnO may bedeposited or grown by one or more of the following processes, epitaxy,sputtering, atomic layer deposition (ALD), metal organic chemical vapordeposition (MOCVD), etc. The carrier concentration (electron density) ofthe layer 18 may be between about 1×10²¹ cm⁻³ to about 5×10²¹ cm³, andpreferably about 3.0×10²¹ cm⁻³ for doped Aluminum Zinc Oxide (ZnO:Al)(AZO).

The n-type material (e.g., ZnO:Al) is preferably crystalline in form.This includes a monocrystalline structure and may include amulti-crystal structure or other crystalline structure (micro, nano,etc.). However, the AZO material may also include amorphous phases. Inone embodiment, the ZnO of layer 18 is amorphous. The underlying layersare also preferably crystalline, but may include other phases.

Referring to FIG. 2, a plot of resistivity (Ohm-cm²) versus length(microns) in a TLM (transmission line measurement) pattern isillustratively depicted to demonstrate resistive properties of then-type layer 18 (FIG. 1) in accordance with the present principles.Resistivity is plotted for AZO material.

A conventional photodiode having an ohmic contact including Ti/Pd/Au onan n+ InGaAs n-type material includes a contact resistivity of, e.g.,greater than 5×10⁻⁸ Ohm-cm². In accordance with the present principles,the AZO contact (n-type layer 18) provides a lower contact resistivity(e.g., about 1.3×10⁻⁹ Ohm-cm²), more than one order of magnitudeimprovement. The conventional photodiode having an ohmic contactincluding Ti/Pd/Au often needs to be formed in a metal grid pattern topermit light transmission and is needed to increase conductivity on ann+ InGaAs n-type material for a conventional device. This metal gridprevents light transmission (forms shadow areas).

In accordance with the present principles, a single layer of n-typematerial is employed that provides a sufficient amount of conductivityand is thinner than the conventional n-type structures. The single layeremitter does not need a metal grid since, e.g., doped ZnO provides ahighly conductive layer.

Referring to FIG. 3, a plot of absolute value of current (mA) versusvoltage (V) is illustratively depicted to compare a conventionalphotodiode with a photodiode in accordance with the present principles.A graph 102 shows an I-V curve for a conventional photodiode having ann+ InGaAs layer as an n-type emitter. A graph 104 shows an I-V curve fora photodiode having an n+ AZO layer in accordance with the presentprinciples. Graph 104 shows dark current (e.g., at V=−1.0 V) being threeorders of magnitude less than the dark current of graph 102. Inaddition, graph 102 provides an on/off ratio of about 1×10³. Graph 104provides an on/off ratio of about 10⁷. The on/off ratio of the device ofgraph 104 is improved by about 4 orders of magnitude over that of thedevice for plot graph 102. The on/off ratio provides an indication ofleakage current and may be defined as the current at 1 V divided by thecurrent at −1 V.

Referring to FIG. 4, a method for forming a photodiode is illustrativelyshown in accordance with the present principles. In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In block 202, a photodiode structure is provided. This includes thepreparation and doping of a substrate (e.g., a p-type substrate and inparticular a p+ doped substrate). The substrate includes a III-Vmaterial such as InP or other suitable material. A p-type ohmic contactis formed on the substrate. The p-type ohmic contact may include ap-type metal or other highly conductive p-type material. An intrinsiclayer is formed over the substrate opposite the p-type ohmic contact.The intrinsic layer includes an undoped or lightly doped III-V material.In one particularly useful embodiment, the intrinsic layer includesInGaAs and in particular, In₅₃Ga₄₇As.

In block 204, a transparent II-VI n-type layer is formed on theintrinsic layer to function as at least an emitter and an n-type ohmiccontact. The II-VI n-type layer may include ZnO, and in particularZnO:Al. The II-VI n-type layer is preferably less than 150 nm inthickness, and more preferably about 100 nm (or less). The II-VI n-typelayer is a single layer emitter that replaces two or three components orlayers of a conventional structure.

In block 206, the transparent II-VI n-type layer provides a dark currentthat is less than a device with a III-V emitter layer. In particularlyuseful embodiments, the dark current can be about one to three (or more)orders of magnitude less than a device with a III-V emitter layer. Inblock 208, the transparent II-VI n-type layer provides a reduced contactresistivity than a device with a III-V emitter layer and a metal ohmiccontact.

Resistivity improvement can be one order of magnitude (or greater) thandevice with a III-V emitter layer and a metal ohmic contact. In block210, the transparent II-VI n-type layer can be formed with an on/offvoltage ratio of about 10⁷ or greater. It should be understood that thephotodiode may include additional components that provide otherfunctions, such as buffer layers, multiple active layers, etc.

Having described preferred embodiments of highly responsive III-Vphotodetectors using ZnO:Al as n-type emitter (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A photodiode comprising: a p-type ohmic contact; a p-type substratein contact with the p-type ohmic contact; an intrinsic layer formed overthe substrate and including a III-V material; and a transparent II-VIn-type layer formed on the intrinsic layer and functioning as at leastan emitter and an n-type ohmic contact.
 2. The photodiode as recited inclaim 1, wherein the II-VI n-type layer includes ZnO.
 3. The photodiodeas recited in claim 1, wherein the ZnO is Al doped.
 4. The photodiode asrecited in claim 1, wherein the II-VI n-type layer is less than 150 nmin thickness.
 5. The photodiode as recited in claim 1, wherein the II-VIn-type layer is less than about 100 nm in thickness.
 6. The photodiodeas recited in claim 1, wherein the intrinsic layer includes InGaAs. 7.The photodiode as recited in claim 1, further comprising a dark currentabout three orders of magnitude less than a device with a III-V emitterlayer.
 8. The photodiode as recited in claim 1, wherein the II-VI n-typelayer includes a reduced contact resistivity of one order of magnitudeor greater than a device with a III-V emitter layer and a metal ohmiccontact.
 9. The photodiode as recited in claim 1, further comprising anon/off voltage ratio of about 10⁷.
 10. A photodiode comprising: a p-typeohmic contact; a p+ III-V substrate in contact with the p-type ohmiccontact; a III-V intrinsic layer formed over the substrate; and a singlelayer emitter formed on the intrinsic layer and including an Al dopedZnO and functioning as at least an emitter and an n-type ohmic contact,the Al doped ZnO including a thickness of less than 150 nm.
 11. Thephotodiode as recited in claim 10, wherein the Al doped ZnO is less thanabout 100 nm in thickness.
 12. The photodiode as recited in claim 10,further comprising a dark current about three orders of magnitude lessthan a device with a III-V emitter layer.
 13. The photodiode as recitedin claim 10, wherein the Al doped ZnO includes a reduced contactresistivity of one order of magnitude or greater than a device with aIII-V emitter layer and a metal ohmic contact.
 14. The photodiode asrecited in claim 10, further comprising an on/off voltage ratio of about10⁷.
 15. A method for forming a photodiode, comprising: providing ap-type ohmic contact, a p-type substrate in contact with the p-typeohmic contact and an intrinsic layer formed over the substrate andincluding a III-V material; and forming a transparent II-VI n-type layeron the intrinsic layer to function as at least an emitter and an n-typeohmic contact.
 16. The method as recited in claim 15, wherein the II-VIn-type layer includes ZnO.
 17. The method as recited in claim 15,wherein the II-VI n-type layer is less than 150 nm in thickness.
 18. Themethod as recited in claim 15, wherein forming includes forming thetransparent II-VI n-type layer with a dark current of about three ordersof magnitude less than a device with a III-V emitter layer.
 19. Themethod as recited in claim 15, wherein forming includes forming thetransparent II-VI n-type layer with a reduced contact resistivity of oneorder of magnitude or greater than a device with a III-V emitter layerand a metal ohmic contact.
 20. The method as recited in claim 15,wherein forming includes forming the transparent II-VI n-type layer withan on/off voltage ratio of about 10⁷.